1. Technical Field
This invention relates to rubber compositions for use in automobile tires and more particularly to such a composition having utility as tire treads of an all-weather type.
2. Prior Art
Automotive vehicles have recently been designed with higher degrees of sophistication particularly as is in the case with passenger cars. This literally requires greater driveability, meaning that tire requirements have become more strict with respect to running stability and braking ability characteristics among other important qualities.
It is known that the above qualities can be improved by the application of rubbers of a large loss tangent to tread portions. Loss tangent is referred to usually as tan .delta. and associated closely with road gripping. The larger loss tangent at the same level of hardness, the more intensely the tire grips the road surface. Great magnitudes of loss tangents are attainable generally with the use of styrene-butadiene rubbers (SBRs) of higher than -40.degree. C. in glass transition temperature (Tg) that are abundant in styrene and vinyl contents. SBRs of such a high Tg depend widely upon temperature and thus vary in hardness over a wide range of temperatures, say from about -20.degree. to 100.degree. C., to which the tire usually gets exposed in actual use. In this instance SBRs when set to maximize tan .delta. at room temperature are encountered with instable runnability not only in cold season but even in warm season due to a sharp rise in temperature during the running of the tire.
Carbon blacks of too small a particle size will in most cases make the resulting rubber mix excessively hard and hence instably runnable. Process oils may be added to reduce or eliminate this problem but to an extent to invite quality deterioration.
Viewed in that situation of the prior art, there has still much to be desired for the provision of tread rubbers of better qualities.